A nitride semiconductor including nitrogen (N) as a Group V element is a prime candidate for a material to make a short-wave light-emitting element because its bandgap is sufficiently wide. Among other things, gallium nitride based compound semiconductors (which will be referred to herein as “GaN-based semiconductors”) have been researched and developed particularly extensively. As a result, blue-ray-emitting light-emitting diodes (LEDs), green-ray-emitting LEDs and semiconductor laser diodes made of GaN-based semiconductors have already been used in actual products.
A gallium nitride-based semiconductor has a wurtzite crystal structure. FIG. 1 schematically illustrates a unit cell of GaN. In an AlaGabIncN (where 0≦a, b, c≦1 and a+b+c=1) semiconductor crystal, some of the Ga atoms shown in FIG. 1 may be replaced with Al and/or In atoms.
FIG. 2 shows four primitive vectors a1, a2, a3 and c, which are generally used to represent planes of a wurtzite crystal structure with four indices (i.e., hexagonal indices). The primitive vector c runs in the [0001] direction, which is called a “c axis”. A plane that intersects with the c axis at right angles is called either a “c plane” or a “(0001) plane”. It should be noted that the “c axis” and the “c plane” are sometimes referred to as “C axis” and “C plane”.
As shown in FIG. 3, the wurtzite crystal structure has other representative crystallographic plane orientations, not just the c plane. Portions (a), (b), (c) and (d) of FIG. 3 illustrate a (0001) plane, a (10-10) plane, a (11-20) plane, and a (10-12) plane, respectively. In this case, “−” attached on the left-hand side of a Miller-Bravais index in the parentheses means a “bar” (a negative direction index). The (0001), (10-10), (11-20) and (10-12) planes are c, m, a and r planes, respectively. The m and a planes are “non-polar planes” that are parallel to the c axis but the r plane is a “semi-polar plane”. It should be noted that the m plane is a generic term that collectively refers to a family of (10-10), (−1010), (1-100), (−1100), (01-10) and (0-110) planes.
Light-emitting elements that use gallium nitride based compound semiconductors have long been made by “c-plane growth” process. In this description, the “X-plane growth” means epitaxial growth that is produced perpendicularly to the X plane (where X==c, m, a or r, for example) of a hexagonal wurtzite structure. As for the X-plane growth, the X plane will be sometimes referred to herein as a “growing plane”. Furthermore, a layer of semiconductor crystals that have been formed as a result of the X-plane growth will be sometimes referred to herein as an “X-plane semiconductor layer”.
If a light-emitting element is fabricated as a semiconductor multilayer structure by c-plane growth process, then intense internal electric polarization will be produced perpendicularly to the c plane (i.e., in the c axis direction) because the c plane is a polar plane. Specifically, that electric polarization is produced because on the c-plane, Ga and N atoms are located at different positions with respect to the c axis. Once such electric polarization is produced in a light-emitting portion, the quantum confinement Stark effect of carriers will be generated. As a result, the probability of radiative recombination of carriers in the light-emitting portion decreases, thus decreasing the light-emitting efficiency as well.
To overcome such a problem, a lot of people have recently been making every effort to grow gallium nitride based compound semiconductors on a non-polar plane such as an m or a plane or on a semi-polar plane such as an r plane. If a non-polar plane can be selected as a growing plane, then no electric polarization will be produced in the thickness direction of the light-emitting portion (i.e., in the crystal growing direction), and therefore, no quantum confinement Stark effect will be generated, either. Thus, a light-emitting element with potentially high efficiency can be fabricated. The same can be said even if a semi-polar plane is selected as a growing plane. That is to say, the influence of the quantum confinement Stark effect can be reduced significantly in that case, too.
FIG. 4(a) schematically illustrates the crystal structure of a nitride-based semiconductor, of which the principal surface is an m plane, as viewed on a cross section thereof that intersects with the principal surface of the substrate at right angles. On the m plane, Ga atoms and nitrogen atoms are on the same atomic plane. For that reason, no electric polarization will be produced perpendicularly to the m plane. It should be noted that In and Al atoms that have been added are located at Ga sites to replace Ga atoms. Even when at least some of the Ga atoms are replaced with In and Al atoms, no electric polarization will be produced perpendicularly to the m plane, either.
The crystal structure of a nitride-based semiconductor, of which the principal surface is a c plane, as viewed on a cross section thereof that intersects with the principal surface of the substrate at right angles is illustrated schematically in FIG. 4(b) just for your reference. In this case, Ga atoms and nitrogen atoms are not present on the same atomic plane, and therefore, electric polarization will be produced perpendicularly to the c plane. A c-plane GaN-based substrate is generally used as a substrate to grow GaN based semiconductor crystals thereon. As the positions of the Ga (or In) atomic layer and nitrogen atomic layer, which are parallel to the c plane, slightly shift from each other in the c-axis direction, electric polarization is produced in the c-axis direction.